Internal combustion engine emission regulations are becoming increasingly stringent. Using multiple combustion modes in internal combustion engines has been considered as a means to reduce emissions and to simultaneously maintain the functionality of the exhaust treatment systems. More sophisticated alternative combustion modes such as low temperature combustion (LTC), homogeneous charge compression ignition (HCCI), and premixed charge compression ignition (PCCI) are being developed and implemented on these engines along with conventional combustion in an attempt to meet emission regulations while maintaining desired performance levels.
Internal combustion engines may be understood to include both spark ignition (including direct ignition) and compression ignition (e.g., diesel) engines. Direct ignition may be understood as a type of spark ignition where each cylinder has a dedicated coil rather than energy from a single coil distributed to multiple cylinders. LTC may be understood to include combustion with a flame local temperature in the range of about 1400° K to about 2200° K (as compared to combustion with a flame local temperature in the range of about 1800° K to about 3600° K). PCCI may be understood to include ignition where fuel and air may be mixed prior to injection into a combustion chamber. HCCI may be understood to be a type of PCCI where the fuel and air mixture is relatively homogeneous at the time of ignition. It may be appreciated that LTC may produce relatively lower levels of nitrogen oxides (NOx) and particulate matter than relatively higher temperature combustion. NOx may be understood to include NO and NO2. As used herein, “about” may be understood to mean±10%.
In addition to alternative combustion modes, it is contemplated that alternative and/or additional fuels may also be used to optimize performance while reducing or maintaining engine-out emissions. For example, in addition to hydrocarbon fuels such as gasoline or diesel fuel. It may also include biodiesel (a diesel-equivalent processed fuel derived from biological sources such as vegetable oils) as well as other alternative fuels which are capable of combustion.
To reduce the engine-out emissions, exhaust gas recirculation (EGR) has been increasingly implemented on internal combustion engines. For some alternative combustion modes such as PCCI and HCCI, a dual-loop EGR system (e.g., a high-pressure loop and a low-pressure loop) may be desirable in order to offer a sufficient amount of EGR gas to control engine-out emissions. The properties of the EGR gases from the high-pressure loop and low-pressure loop may be different due to cooling and filtering effects. For example, the low-pressure loop may receive exhaust gas from downstream of a turbocharger and after-treatment so that such gas is relatively cool and relatively clean. The high-pressure loop, on the other hand, may receive exhaust gas from upstream of the turbocharger and after-treatment so that such gas is relatively warm and relatively unclean. Some engines may have both a low-pressure loop EGR and a high-pressure loop EGR system.
Dual-loop EGR systems may also enhance the operation of alternative combustion engines. Emission and control requirements of alternative combustion engines may vary with operating conditions, e.g., speed-load relationship. For example, the relatively higher temperature HP-EGR gas may help stabilize combustion for relatively light or low load conditions. Similarly, the relatively clean, filtered LP-EGR gas with high mass air flow rate may help reduce NOx emissions for relatively heavy or high load conditions.
The dual-loop EGR system may introduce challenges for an engine control system because of a lack of measurement of the EGR/air rate in the intake manifold and a lack of measurement of the ratio of high-pressure EGR gas to low-pressure EGR gas. It may therefore be desirable to estimate the air fractions in all intake and exhaust sections of an engine in order to provide closed-loop control of the EGR rate, high-pressure EGR amount and low-pressure EGR amount. Air fraction may be understood to mean the fraction of a measured gas that is air. An air fraction of one may correspond to 100% air and an air fraction of zero may correspond to 0% air.
For example, for diesel engines, combustion is generally complete and the air to fuel ratio is typically lean. Complete combustion may be understood to mean substantially all of the fuel is burned so no unburned fuel remains. Air to fuel ratio (AFR or λ) may be defined as the mass ratio of air to fuel present during combustion. As used herein, both AFR and λ may be understood to mean the air to fuel ratio.
A stoichiometric mixture may be defined as chemically balanced, i.e., all the fuel is combined with all the oxygen. If the air-fuel ratio is greater than the stoichiometric amount, the mixture may be said to be lean, i.e., more air than fuel. Likewise, if the air-fuel ratio is less than the stoichiometric amount, the mixture may be said to be rich, i.e., more fuel than air. Accordingly, in a diesel engine, the air to fuel ratio is typically greater than the stoichiometric amount, i.e., lean. The exhaust gas may, therefore, contain unburned air that may be returned or recirculated to an intake manifold through one or more EGR systems. Because the combustion is generally complete, the exhaust gas may not contain unburned fuel.